Automatic selectivity control



April 25, 1939. j BEGGS 2,156,077

AUTOMATIC SELECTIVITY CONTROL Filed Oct. 22, 51.936

James ELBe gs,

His Attorney.

Patented Apr. 25, 1939 AUTOMATIC SELECTiVITY CONTROL James E. Beggs, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application October 22, 1936, Serial No. 106,977

7 Claims.

My invention relates to resonant networks and particularly to such networks which are employed in radio receiving systems. It is well known that when relatively weak signals are being received by such systems greater selectivity may be obtained by limiting the frequencies received to a narrow band and that when the signals are relatively strong the band of received frequencies may be broadened out thereby improving the lidelity of the reception although this may be at some expense of selectivity.

One ob-ject'of my invention is the provision of such a network having improved means for automatically changing the width of the band of received frequencies in accordance with the signal strength. Another object of my invention is the provision of improved apparatus of that character having means for producing a phase displaced voltage to vary the band width and a single electron discharge amplifier operative simultaneously to amplify the signal and to control said voltage.

My invention will be better understood from the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Referring to the drawing, Fig. 1 is a circuit diagram illustrating a preferred embodiment of my invention; Fig. 2 is a cross-sectional view of an electron discharge amplifier forming a part of the apparatus shown in Fig. 1; and Fig. 3 shows resonance curves obtained with the apparatus shown in Fig. 1 under various conditions of signal strength.

In Fig. 1 I have shown my invention as forming a part of a conventional superheterodyne radio receiving system in which I is the antenna or pick-up device, 2 is the radio frequency amplifier stage, 3 is the detector and oscillator stage, 4 is a stage of intermediate frequency amplification, 5 is a second detector stage, 6 an audio frequency amplifier stage, and l is a loudspeaker. The stage 4 comprises the electron dischargeamplifier 9 which is of special construction to be described later, which amplifier is coupled to stage 3 by the coupling transformer ID. The primary and secondary windings Hand [2 respectively of this transformer are wound coaxially and in the same direction, the circuit of each winding being tuned to the intermediate frequency by means of the capacitors l3 and M.

In accordance with my invention I have provided means for increasing the elfective coupling of the transformer to increase the width of the band of frequencies passed by the transformer in response to an increase in the strength of the received signals. For the purpose of thus increasing the effective coupling of the transformer I employ the phase shifting network l6 which is connected with the primary of the transformer l0 so as to apply thereto a voltage which is substaniv tially in quadrature with and ahead of the signal voltage applied to the primary from the previous stage. The effectiveness of this network is controlled by the amplifier 9 which functions both to amplify the signal in the usual manner subject to an automatic volume control voltage and also to amplify the signal in an inverse manner for the control of the phase shifting network Hi.

This network comprises the capacitor l1 and the resistor 18 connected in series across the primary H, the capacitor being arranged adjacent to the upper end of the primary and the resistor being connected with the lower end of the primary and with ground through the blocking capacitor 20. The impedance of the capacitor is much greater than that of the resistor at the fre' quency employed whereby the voltage applied to the primary by the capacitor is shifted nearly ahead of the voltage applied to the primary by the previous stage. Connected across the resistor I8 is the short-circuiting switch I 9 whose purpose will be explained later.

The amplifier 9 has the cathode 22 which is connected with ground through the self-biasing resistor 23, the main anode 24, the control grid 25, the screen grid 26, the gain control grid 2! and the auxiliary anode 28. The latter anode, as more clearly shown in Fig. 2, comprises the two rods which are arranged between one part of the screen grid 26 and the gain control grid 21'. The control grid 25 connects with upper end of the secondary l2. The main anode 24 connects in the usual manner with the detector stage 5 whereby the amplified signals are passed on to that stage. The auxiliary anode connects through the conductor 29 with a point on the phase shifting network It between the capacitor and the resistor whereby the amplifier 9 supplies an amplified signal voltage to the network. Since the gain control grid 2'! is arranged between the main anode and the auxiliary anode the eifect of an increased negative charge thereon is to cause the electrons emitted from the cathode to be deflected fromthe main anode toward the auxiliary 1 anode. In the operation of the ampifier 9 therefore amplification-of the signal in the one anode circuit is the reverse of that of the other anode circuit; hence when the signal strength increases the resulting increase in the negative automatic when the received signal is weak, at which time a high degree of selectivity is more desirable than a high degree of fidelity, the automatic volume control voltage supplied to the gain control grid 21 will be a minimum. The main output of the amplifier then will be through the main amplifier section which includes the main anode 24, the output of the auxiliary anode being at that time small since a relatively few electrons reach that anode. Since the voltage supplied to the network l6 by the auxiliary anode of the amplifier is small, the phase shifted voltage applied to the primary by the network is accordingly small; hence the increase in the effective coupling of the transformer and the resulting band spread effect is small. With a weak signal, therefore, the resonance curve may have the form shown at 30 of Fig. 3.

When the received signal increases, the resultant increase in the automatic volume control voltage applied to the gain control grid decreases the output of the amplifier through the main amplifier section but causes an increase in the output of the auxiliary anode or band spread section since the gain control grid by becoming more negative causes electrons formerly reaching the main anode to be turned back to the auxiliary anode. The greater voltage now supplied to the phase shifting network results in a greater phase displaced voltage being supplied to the primary of the transformer l0. Hence the effective coupling thereof is increased and the band spread widened. With a strong signal, therefore, the resonance curve may take a form such as shown at 3| in Fig. 3.

For the above-described operation it is understood that the short-circuiting switch I9 is in open position. Should one desire to obtain a still greater degree of selectivity when the signals are weak than that represented by the curve 30, the switch l9 may be closed thereby short-circuiting the resistor N3 in order to decrease the effect of the small signal voltage received from the auxiliary anode at that time. With the switch l9 closed the resonance curve may take the form shown by the curve 32 of Fig. 3.

I have chosen the particular embodiment described above as illustrative of my invention and it will be apparent that various modifications may be made without departing from the spirit and scope of my invention which modifications I aim to cover by the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a plurality of coupled circuits tuned to the same frequency arranged to transmit a signal, means for varying the effective coupling between said circuits comprising a phase shifting network associated with one of said circuits and a single electron discharge device having means for amplifying the signal voltage and separate means for supplying feedback voltage to said phase shifting network.

2. In combination, a plurality of coupled circuits tuned to the same frequency arranged to ttransmit a signal, means for varying the effective coupling between said circuits comprising a phase shifting network associated with one of said circuits and a single electron discharge device for amplifying the transmitted signal and for supplying a separate feedback voltage to said phase shifting network, said device having means responsive to an increase inthe signal for causing a decrease in the amplification thereof and an increase in the feedback voltage.

3. In combination, a plurality of coupled circuits tuned to the same frequency arranged to transmit a signal, means for supplying a signal to one of said circuits, an electron discharge amplifier device connected with the other of said circuits, said device having a main anode for obtaining the amplified signal and an auxiliary anode for obtaining a feedback voltage, a phase shifting network connected with said one circuit and means connecting said network with said auxiliary anode.

4. In combination, a plurality of coupled circuits tuned to the same frequency arranged to transmit a signal, a phase shifting network associated with one of said circuits, an electron discharge amplifier device associated with the other of said circuits having a main anode, an

auxiliary anode and a grid therebetween, means connecting said auxiliary anode with said network and means responsive to the amplitude of the signal for supplying a negative bias to said grid.

5. In combination, a transformer having primary and secondary windings tuned to the same frequency for transmitting a signal, a phase shifting network connected with said primary and a single electron discharge device connected with said secondary arranged to amplify said signal andto supply a separate feedback voltage for said network said device having means responsive to an increase in the signal for causing less amplification of the signal thereby and a greater feedback voltage.

6. In combination, a multi-electrode amplifier including a cathode, a first control electrode and a main anode arranged in the order named, an input circuit coupled between the cathode and the first control grid for impressing a signal thereon, a second control grid arranged between the first control grid and the main anode, means for impressing on the second grid a negative bias proportional to the signal strength, an auxiliary anode interposed between the first and second control grids for collecting a current proportional to the negative bias impressed on the second control grid, and a connection between said auxiliary anode and said input circuit of such character as to exert a regulatory influence on the signal input to the amplifier.

7. In combination a source of electric waves, a circuit tobe energized in accordance with changes in a particular characteristic of said waves, ,an electron discharge amplifier for producing an electronic current corresponding to said waves, said amplifier including a cathode and a first control electrode adapted to be influenced by said source, a main anode for receiving the said-electronic current and for energizing a primary consumption circuit therewith, an auxiliary anode arranged between the first control l trode'and the main anode, said auxiliary anode being connected to supply current to the circuit to be energized, a second control grid arranged between the main and auxiliary anodes and adapted to divert a variable current to the latter in accordance with variations in the bias impressed on such grid, thereby to effect similar variations in the current supplied to the circuit to be energized, and means for adjusting the bias of the second control grid in accordance with changes in the said particular characteristic of said waves.

JAMES E. BEGGS. 

